I think what you really need to do is figure out what you want in your core components and then build around it. I'd spend the most on a GPU. An 8800 GT 512 MB or 3870 512 MB should hold you over for a couple to even a few years of gaming/simming. Expect that to run at least $200 on a deal. Secondary is CPU, and a dual core for the future will be a must. Next is RAM, which needs to be 2 GB minimum.
If you do not care about your old system, you might cannabilize the case, possible the PSU, but I doubt your old PSU will power a new system these days, but its worth a look. Make sure any PSU you do buy has at least 30 AMps on the +12 volt rail, so you won't have a problem running at least 1 modern GPU like the two mentioned above. Also, you might consider cannabilizing the optical drives, and the hard drives to hold you over till you buy new.
Last look at motherboard + any little extras you may need like case fans and a decent CPU cooler.
Figuring that out - $200 GPU + $150 CPU + $125 motherboard + $50 odds and ends + assume you cannabilize your old system to the max, and you have a base. Your base is $525 at that point. This leaves you $275 to buy anything from a new hard drive (saw Seagate SATA 300 GB with 16 MB cache for only $69 yesterday), a decent case for $50, a PSU for $75, DVD-Burner for $30. I see deals for $30 for 2 GB of fast DDR2800 RAM too.
The point is - $800 is very doable for a pretty fast system that will even play a game like Crysis, let alone SHIV.
-S
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